1. Field of the Invention
The invention relates to a liquid transfer device for a fluid analysis unit. The liquid transfer device has a liquid transfer needle and a capacitive liquid level detector, for detecting the submersion or dipping of the liquid transfer needle into an analysis liquid contained in a vessel. The liquid level detector includes two electrodes and a detection circuit with an a.c. voltage source for detecting a change in capacitance between the electrodes.
2. Description of the Related Art
In analysis units of the type necessary for the analysis of body fluids such as blood, liquid transfer devices are required in order to transfer analysis liquids, such as liquid samples or reagents. Common liquid transfer devices are, for example, pipettes, which are used for removing samples or reagents from a first vessel and ejecting them into a second vessel. Another common liquid transfer device is a dispenser in which the liquid transfer needle is connected via a hose to a greater stock of the liquid, which may be ejected through the needles by means of a pump device. Dispensers usually also perform the function as a pipette. A liquid transfer device for the purpose of the present invention is essentially any device or structure which in an analysis unit serves to dip into an analysis liquid in order to make possible some liquid transfer steps (intaking and/or ejection of liquid) by means of the liquid transfer needle. The liquid transfer needle is a hollow needle which usually consists of a thin tube of metal or plastic. It will be referred to herein, for the sake of simplicity, as the "needle".
If the needle is dipped or submerged deep into the analysis liquid, a relatively large amount of excess liquid remains undesirably suspended in and around the needle. The amount of liquid transfer can thereby be inaccurate, and during the next dip of the needle, the excess liquid contaminates the remaining analysis liquid (so-called "carry over") In order to better monitor the dipping depth, liquid transfer devices are provided with a sensor device for. detecting the dipping of the needle into the analysis liquid, which is usually referred to as a liquid level detector or LLD. The liquid level detector is connected to a vertical drive by means of which the needle is dipped into the analysis liquid, in order to arrest the dipping movement when the tip of the needle is dipped in the analysis liquid by a few millimeters. The vertical position of the needle acts as a measure of the level of the liquid in the respective vessel. Consequently, the liquid level detector makes it possible to monitor the amount of liquid present in the respective vessel, in order to give a signal when the stock of a reagent liquid is depleted and the reagent bottle therefore has to be replaced.
A commonly used construction principle for liquid level detectors is based on measuring the electrical resistance between the needle and an electrode attached to the tip of the needle. Needle and electrode are insulated electrically from one another, so that the electrical resistance between them in the dry state is very high. Upon dipping the needle and the electrode into a sample liquid, the sample liquid closes the circuit so that the electrical resistance changes suddenly. The resulting signal can be detected reliably with simple electronic means. It is regarded as a major disadvantage of this principle, however, that an electrode must be dipped into the liquid in addition to the needle, which increases the amount of excess liquid which unavoidably remains suspended thereupon. The previously mentioned problems with regard to carryover and reduced accuracy are therefore further exacerbated.
Superior detectors for this purpose are liquid level detectors in which a change in the electrical capacitance between two sensor electrodes is recorded as a signal for detecting the dipping of the needle into the liquid. The signal is recorded by an electronic detection circuit coupled to the sensor electrodes, which includes an a.c. voltage source. The first electrode is usually the needle, which can consist of metal or an electrically conductive (metallized) plastic material; the hot pole of the a.c. voltage source is connected thereto, thereby forming a signal electrode. The counter electrode, which is usually connected to earth potential or ground, can be arranged on the outside of the liquid vessel, such as below the bottom and partially around the side walls of the vessel. It is usually an integral part of the vessel support. Upon dipping the needle tip into the liquid, the capacitance between the signal electrode and the counter electrode changes on the basis of the electrical conductivity and the dielectric properties of the liquid.
Liquid level detectors of the kind are described in EP-A No. 0 164 679, U.S. Pat. No. 4,818,492, and EP-A No. 0 355 791, which are hereby incorporated by reference. These printed publications contain more detailed explanations, to which reference will be made here.
A basic problem with these capacitive liquid level detectors exists in the fact that the change in capacitance upon the dipping into the liquid is very small in comparison with the other capacitances which are inherently present, such as "noise capacitances" from, for example, the connecting cable and the amplifier input. The ratio of the useful signal to the noise signals is consequently highly unsatisfactory. Particularly problematic is the fact that some of the noise capacitances are not constant, but change relatively rapidly with time. This is particularly applicable to capacitive interference, which is caused by the movement of objects, such as component parts of the automatic analysis unit, and hands or other parts of the body of the operating personnel. On a fully automatic analysis unit which has numerous moving parts, such interference is impossible to avoid in practice.
In EP-A No. 0 355 791, a specific problem of this kind (noise caused by a membrane sealing the vessel) is corrected by the fact that a reference signal is fixed upon the contacting of the membrane, and during the further downward movement of the needle the difference relative to the fixed reference signal is detected. However, this method is directed towards this specific application. Noise capacitances which change between the fixing of the reference signal and the detection of the liquid surface can often lead to faulty detection.
In the case of the liquid level detector described in the U.S. Pat. No. 4,818,492, the noise capacitance of the leads is compensated passively by means of a bridge circuit. Other capacitive noise is not thereby eliminated, however, and can similarly cause faulty detection with this type of detector.